Embodiments of the invention relate to a method for increasing the maximum offset distance for underwater sensors compliant with subsea instrumentation interface standard (SIIS) Level 2, for example CANbus Fault Tolerant, connected to subsea modules, for example subsea control modules (SCMs) or power and communication distribution modules (PCDMs). A sensor bus which implements the method is also claimed.
SIIS is the Subsea Instrumentation Interface Standard being defined and in use by the offshore oil and gas Industry. It seeks to create a common platform for implementation of subsea sensors. SIIS Level 2 applies to a family of digital serial devices. The current implementation of SIIS Level 2 is a CANbus Fault Tolerant physical Layer (with CANOpen protocol).
The implementation of a SIIS Level 2 sensor network is impacted by a number of parameters, such as: number of sensor nodes, baud rate, bit sampling point, cable capacitance, and termination resistance.
These parameters constrain the maximum offset distance the sensor may reach away from the subsea control module (or similar subsea module). The ‘offset distance’ is the length of the cables connecting a sensor to the subsea control module. In a star network topology this is equivalent to the sum of the length of cable from the sensor to the cable split plus the length of cable from the cable split to the subsea control module.
Each parameter is considered in turn below: